During recent years, in the test-tube manufacturing sector, glass has been gradually abandoned and there has been an increasing use of plastic polymers such as, for example, polyethylene terephthalate (PET), polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polyurethane (PUR), polystyrene (PS) or olefin and ethylene-vinyl alcohol (EVOH) copolymers.
Compared to glass, materials such as PET, PP or PE offer undoubted advantages from a processing point of view and from the point of view of the mechanical properties which are imparted to the test tubes.
Test tubes made with these materials are in fact lighter, more resistant to impacts and, in particular, less costly than glass test tubes, since, owing to the greater ease of using injection moulding techniques, the production time and costs may be reduced considerably.
In general, test tubes for taking blood samples must be able to maintain, for a predetermined minimum life span, a well-defined blood intake capacity by ensuring that there is a certain degree of vacuum still present in the test tube compared to the vacuum pre-set at the time of sealing thereof. This capacity is measured in the form of the intake blood volume and corresponds to a fraction of the nominal volume V of the test tube. This fraction will be referred to below as the intake vacuum volume V2. The duration of the life span of a test tube is closely linked to the permeability of the walls of the said test tube and therefore the gas barrier properties of the materials used.
As is known, test tubes for taking blood samples which are specifically intended for clinical tests for measurement of the coagulation or for measuring the erythrocyte sedimentation rate (ESR) contain inside them a predetermined quantity of an anti-coagulating substance (usually an aqueous solution of sodium citrate) which at the time when the blood sample is taken is mixed with the blood, preventing coagulation thereof, and therefore allows the examination to be carried out at a later time. For the coagulation test it is at present envisaged that the anti-coagulating solution is mixed with the blood in a volumetric ratio of about 1:9 (=0.111), while for measurement of the ESR it is currently envisaged that the anti-coagulating solution is mixed with the blood in a ratio of about 1:4 (=0.25).
Operationally speaking, the inside of the test tubes intended for these tests must therefore usually have a ratio V1/V2 of about 1:9 or about 1:4 between the volume of anti-coagulating solution V1 and the intake vacuum volume V2. The sum of the intake vacuum volume V2 and the anti-coagulant volume V1 defines the working volume Vu of the test tube which is established a priori by the manufacturer. The working volume Vu, which in most cases is fixed between 1.5 and 4.5 ml, occupies only a fraction of the nominal volume V of the said test tube, while the remaining fraction, defined as the mixing volume Vm, is left free to allow mixing of the blood with the anti-coagulant.
Obviously, in order to achieve the volumetric ratio of 1:9, the anti-coagulant volume and the intake vacuum volume must occupy 1/10th and 9/10ths, respectively, of the working volume Vu, whereas in order to achieve the ratio of 1:4 they must occupy ⅕th and ⅘ths of the working volume, respectively.
In order for the abovementioned volumetric ratios to be kept within clinically acceptable limits, the test tubes must be able to ensure during the life span defined by the manufacturer that not only the intake vacuum volume, but also the anti-coagulant solution volume are maintained. Therefore, both the liquid (and vapour) barrier properties of the materials used for manufacture of the test tube and the gas barrier properties play a part when determining the life span of the test tube.
With regard to this specific point, the US standards as defined by the NCCLS (National Committee for Clinical Laboratory Standards), which are recognized internationally, fix a maximum tolerance of ±10% for variations, over time, of the vacuum volume V2 and the anti-coagulant volume V1 compared to the abovementioned ideal volumes. Corresponding deviations from the value of the ratio for anti-coagulant volume V1 and intake vacuum volume V2 are therefore permissible.
“Life span” of a test tube must therefore be understood as meaning the period of time for which a test tube manages to ensure variations in the intake vacuum volume and the anti-coagulant volume of less than ±10% of the abovementioned ideal volume values.
At present, the test tubes for the coagulation test or for erythrocyte sedimentation which are made of glass ensure life spans which are considerably longer than those instead ensured by plastic test tubes. In fact, glass test tubes are able to ensure a life span of more than one year compared to the few months of plastic test tubes. For example, in the case of a conventional glass test tube used for coagulation, the manufacturers may even guarantee a life span of 18 months, while, for a similar test tube made of PET with a wall thickness of about 0.9 mm, the manufacturers usually guarantee a maximum life span of 3 to 4 months. In the case of a PET test tube used for erythrocyte sedimentation, a life span of 5-6 months may also be reached.
It is known, in fact, that glass has both excellent gas and liquid barrier properties. At the moment, however, a plastic polymer which combines both these properties with the same efficiency as glass is not known.
Therefore, in a conventional test tube made of plastic, it is inevitable that within the space of a few months an incoming air flow A and outgoing water vapour flow B will be formed such as to cause the intake vacuum volume and the anti-coagulant volume to fall below the permitted limits.
These two flows are regulated by factors which are not yet entirely known and which are closely linked to the physical/chemical structure of the plastic polymers used for manufacture of the test tubes. In particular, in the case of test tubes made of PET, it is noted that the transmission of the vapour outside the test tubes is on average faster than the transmission of the air inside and this results in a more marked decrease in the anticoagulant volume over the vacuum volume.
The limited life spans which may be ensured for plastic tubes are, as already mentioned, in the region of a few months and greatly limit the commercial applicability of plastic test tubes. It should be remembered that on occasions transportation alone may take a few months, as when merchandise is shipped overseas, and that the storage time in warehouses may be prolonged for various reasons beyond the dates planned by the manufacturer. It may therefore happen that the test tubes are delivered to the end user close to the expiry date and therefore must be discarded should they not be used very soon after delivery.
In this connection it is therefore necessary to address the very urgent need in the plastic test tube manufacturing sector to improve the gas and liquid barrier properties of plastic vacuum test tubes in order to increase as far as possible their life spans.
European patent EP 571116 proposes solving this problem by covering externally the conventional plastic test tubes with a special adhesive film consisting of a polymer substrate and a very thin film of compounds based on silicon oxides. The gas and liquid barrier properties are provided mainly by the film of oxides which is deposited on the polymer substrate (formed for example by nylon, PVC, PP, PE, PCTFE or PET) using plasma deposition techniques.
A similar solution is also proposed in European patent EP 603717, which claims an outer, adhesive, protective film consisting of a polymer substrate (PP, PE or PET) and two very thin superimposed films, one of which is formed by a mixture of aluminium oxides and silicon and the other by an organic mixture comprising vinylidene chloride, acrylonitrile, methyl methacrylate, methacrylate and/or acrylic acid copolymers.
The European patents EP 735921 and EP 1175941 solve the problem by combining test tubes one inside the other one, the inner one being made with a polymer material having liquid barrier properties (for example PP) and the outer one being made with a polymer material having gas barrier properties (for example PET).
The abovementioned solutions of the prior art, while solving substantially the problem of the excessive gas and liquid permeability of plastic test tubes, introduce however into the production processes plant- and management-related problems which are often considerable and result in an increase in the production times and costs.
For example, the solutions described in the patents EP 571116 and EP 603717 require the purchase of costly oxide-coated films, while the solutions described in the patents EP 735921 and EP 1175941 require at least diversification of the test tube production lines (for inner test tubes and outer test tubes) and the provision of a final assembly line.